T. gondii is a major cause of neurologic disease in AIDS patients. Currently only one drug regimen has been shown to be effective in treating T. gondii infection, and thus alternative agents are needed. Potential therapeutic agents are screened in standard in vitro culture assays, as well as by an assay specific for folate metabolism in T. gondii that utilizes incorporation of titrated para-aminobenzoic acid into reduce folates. By these techniques, we are in the process of screening agents that interfere with folate metabolism, including those that will inhibit specific enzymes in the folate metabolism pathways. To facilitate such studies, we are also in the process of developing a recombinant DNA library using T. gondii DNA. The goal of such studies would be to identify the genes coding for target enzymes, and to produce such enzymes in large quantities so that specific chemotherapeutic agents can be assayed in vitro prior to proceeding to some of the more cumbersome techniques outlined above. The ultimate goal of these studies is to identify agents that would be alternatives to the currently available regimen for treating T. gondii infection in immunosuppressed patients, including patients with AIDS.